An Operating Systems Vade Mecum

Policies 353C10181220BE03915AD0 5 10 15 20A B C D EFigure 2.7 FCFS scheduleThe following table shows the same information.Process Arrival Service Start FinishT M Pname time required time time A 0 3 0 3 3 0 1.0 B 1 5 3 8 7 2 1.4 C 3 2 8 10 7 5 3.5 D 9 5 10 15 6 1 1.2 E 12 5 15 20 8 3 1.6Mean 6.2 2.2 1.74FCFS is an example of a non-preemptive policy, which means that we neverblock a process once it has started to run until it leaves the domain of the short-termscheduler. (We might terminate a process that has exceeded an initial time estimate, butsuch termination is different from preemption.) The decision of when to run a particularprocess is based solely on the relative order in which it arrives at the ready list.Non-preemptive policies are an example of the Hysteresis Principle, which wewill encounter repeatedly.Hysteresis PrincipleResist change.All change has a cost. In our case, preemption involves switching to a new process,which requires updating software tables and often hardware tables as well. Nonpreemptivescheduling policies avoid the cost of change by resisting process switchinguntil it is inevitable.